In conventional radio frequency (RF) plasma processing, such as is used during stages of fabrication of many semiconductor devices, RF energy, which may be generated in continuous or pulsed wave modes, may be provided to a substrate process chamber via an RF energy source. Due to mismatches between the impedance of the RF energy source and the plasma formed in the process chamber, RF energy is reflected back to the RF energy source, resulting in inefficient use of the RF energy and wasting energy, potential damage to the process chamber or RF energy source, and potential inconsistency/non-repeatability issues with respect to substrate processing. As such, the RF energy is often coupled to the plasma in the process chamber through a fixed or tunable matching network that operates to minimize the reflected RF energy by more closely matching the impedance of the plasma to the impedance of the RF energy source. The matching network ensures that the output of the RF source is efficiently coupled to the plasma to maximize the amount of energy coupled to the plasma (e.g., referred to as tuning the RF power delivery). Thus, the matching network ensures that the total impedance (i.e., plasma impedance+chamber impedance+matching network impedance) is the same as the output impedance of the RF power delivery. In some embodiments, the RF energy source may also be capable of frequency tuning, or adjusting the frequency of the RF energy provided by the RF energy source, in order to assist in impedance matching.
However, in systems where RF generators provide pulsed RF power in the form of a saw tooth wave, the matching network and/or RF energy source cannot be adequately tuned to the best matching position to reduce the reflected power. Thus, in order to adequately reduce reflected power, consistent levels of RF power should be delivered in order for the matching network and/or RF energy source to pick up the signals and tune accordingly. However, standard dual-level pulsing at consistent levels of RF power (i.e., square wave pulsing), plasma intensity changes slowly and the dual-level square wave pulsing may not adequately approximate a saw tooth pattern.
Accordingly, the inventors have provided improved methods and apparatus for RF power delivery.